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A systematic study of variability among OB-stars based on HIPPARCOS photometry
Context: Variability is a key factor for understanding the nature of themost massive stars, the OB stars. Such stars lie closest to the unstableupper limit of star formation. Aims: In terms of statistics, thedata from the HIPPARCOS satellite are unique because of time coverageand uniformity. They are ideal to study variability in this large,uniform sample of OB stars. Methods: We used statisticaltechniques to determine an independant threshold of variabilitycorresponding to our sample of OB stars, and then applied an automaticalgorithm to search for periods in the data of stars that are locatedabove this threshold. We separated the sample stars into 4 maincategories of variability: 3 intrinsic and 1 extrinsic. The intrinsiccategories are: OB main sequence stars (~2/3 of the sample), OBe stars(~10%) and OB Supergiant stars (~1/4).The extrinsic category refers toeclipsing binaries. Results: We classified about 30% of the wholesample as variable, although the fraction depends on magnitude level dueto instrumental limitations. OBe stars tend to be much more variable(≈80%) than the average sample star, while OBMS stars are belowaverage and OBSG stars are average. Types of variables include αCyg, β Cep, slowly pulsating stars and other types from the generalcatalog of variable stars. As for eclipsing binaries, there arerelatively more contact than detached systems among the OBMS and OBestars, and about equal numbers among OBSG stars.
| β Cephei stars in the ASAS-3 data. II. 103 new β Cephei stars and a discussion of low-frequency modes
Context: The β Cephei stars have been studied for over a hundredyears. Despite this, many interesting problems related to this class ofvariable stars remain unsolved. Fortunately, these stars seem to bewell-suited to asteroseismology. Hence, the results of seismic analysisof β Cephei stars should help us to better understand pulsationsand the main sequence evolution of massive stars, particularly theeffect of rotation on mode excitation and internal structure. It istherefore extremely important to increase the sample of known βCephei stars and select targets that are useful for asteroseismology. Aims: We analysed ASAS-3 photometry of bright early-type stars with thegoal of finding new β Cephei stars. We were particularly interestedin β Cephei stars that would be good for seismic analysis, i.e.,stars that (i) have a large number of excited modes; (ii) showrotationally split modes; (iii) are components of eclipsing binarysystems; (iv) have low-frequency modes, that is, are hybrid βCephei/SPB stars. Methods: Our study was made with a homogeneous sampleof over 4100 stars having MK spectral type B5 or earlier. For thesestars, the ASAS-3 photometry was analysed by means of a Fourierperiodogram. Results: We have discovered 103 β Cephei stars,nearly doubling the number of previously known stars of this type. Amongthese stars, four are components of eclipsing binaries, seven have modesequidistant or nearly equidistant in frequency. In addition, we foundfive β Cephei stars that show low-frequency periodic variations,very likely due to pulsations. We therefore regard them as candidatehybrid β Cephei/SPB pulsators. All these stars are potentially veryuseful for seismic modeling. Moreover, we found β Cephei-typepulsations in three late O-type stars and fast period changes in one, HD168050.Table 2 and Figs. 2-14 are only available in electronic form athttp://www.aanda.org The V photometry for all 103 stars is available inelectronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr(130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/477/917
| β Cephei stars in the ASAS-3 data. I. Long-term variations of periods and amplitudes
Aims.We analyse V-filter ASAS-3 photometry of 41 known βCephei-type stars. The ASAS-3 photometry is combined with the archivaldata, if available, to determine long-term stability of periods andamplitudes of excited modes. Methods: Frequencies of modes are derivedby means of Fourier periodograms with consecutive prewhitening. Theresults are examined in the context of detection threshold. Results: Wedetected amplitude changes in three β Cephei stars, BW Cru, V836Cen, and V348 Nor. Period changes were found in KK Vel and V836 Cen. Ouranalysis shows that intrinsic period changes are more common amongmultiperiodic stars, apparently because they are caused by some kind ofmode interaction. In addition, we found new modes for seven stars, andfor ten others we provide new solutions or remove ambiguities in thedetected frequencies. One candidate hybrid β Cephei/SPB star, HD133823, is discovered.Appendix A, Figs. 2-6 and Tables 2-6 are only available in electronicform at http://www.aanda.org Table V photometry for all 41 stars is onlyavailable in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/477/907
| New Estimates of the Solar-Neighborhood Massive Star Birthrate and the Galactic Supernova Rate
The birthrate of stars of masses >=10 Msolar is estimatedfrom a sample of just over 400 O3-B2 dwarfs within 1.5 kpc of the Sunand the result extrapolated to estimate the Galactic supernova ratecontributed by such stars. The solar-neighborhood Galactic-plane massivestar birthrate is estimated at ~176 stars kpc-3Myr-1. On the basis of a model in which the Galactic stellardensity distribution comprises a ``disk+central hole'' like that of thedust infrared emission (as proposed by Drimmel and Spergel), theGalactic supernova rate is estimated at probably not less than ~1 normore than ~2 per century and the number of O3-B2 dwarfs within the solarcircle at ~200,000.
| Catalog of Galactic β Cephei Stars
We present an extensive and up-to-date catalog of Galactic β Cepheistars. This catalog is intended to give a comprehensive overview ofobservational characteristics of all known β Cephei stars, coveringinformation until 2004 June. Ninety-three stars could be confirmed to beβ Cephei stars. We use data from more than 250 papers publishedover the last nearly 100 years, and we provide over 45 notes onindividual stars. For some stars we reanalyzed published data orconducted our own analyses. Sixty-one stars were rejected from the finalβ Cephei list, and 77 stars are suspected to be β Cepheistars. A list of critically selected pulsation frequencies for confirmedβ Cephei stars is also presented.We analyze the β Cephei stars as a group, such as the distributionsof their spectral types, projected rotational velocities, radialvelocities, pulsation periods, and Galactic coordinates. We confirm thatthe majority of the β Cephei stars are multiperiodic pulsators. Weshow that, besides two exceptions, the β Cephei stars with highpulsation amplitudes are slow rotators. Those higher amplitude starshave angular rotational velocities in the same range as thehigh-amplitude δ Scuti stars (Prot>~3 days).We construct a theoretical HR diagram that suggests that almost all 93β Cephei stars are main-sequence objects. We discuss theobservational boundaries of β Cephei pulsation and the physicalparameters of the stars. We corroborate that the excited pulsation modesare near to the radial fundamental mode in frequency and we show thatthe mass distribution of the stars peaks at 12 Msolar. Wepoint out that the theoretical instability strip of the β Cepheistars is filled neither at the cool nor at the hot end and attempt toexplain this observation.
| Catalog of Galactic OB Stars
An all-sky catalog of Galactic OB stars has been created by extendingthe Case-Hamburg Galactic plane luminous-stars surveys to include 5500additional objects drawn from the literature. This work brings the totalnumber of known or reasonably suspected OB stars to over 16,000.Companion databases of UBVβ photometry and MK classifications forthese objects include nearly 30,000 and 20,000 entries, respectively.
| β Cep stars from a spectroscopic point of view
In this review we present the current status of line-profile-variationstudies of β Cep stars. Such studies have been performed for 26bright members of this class of pulsating stars in the past 25 years. Wedescribe all these currently available data and summarize theinterpretations based on them in terms of the excited pulsation modes.We emphasize that line-profile variations offer a much more detailedpicture of the pulsational behaviour of pulsating stars compared toground-based photometric data. The latter, however, remain necessary tounravel the often complex frequency pattern and to achieve unambiguousmode identification for multiperiodic β Cep stars and also toderive the pulsational properties of the faint members of the class. Wehighlight the statistical properties of the sample of 26 stars for whichaccurate spectroscopic studies are available and point out some futureprospects.
| Influence of non-adiabatic temperature variations on line profile variations of slowly rotating beta Cephei stars and SPBs. II. Simulations of line profile time series
We investigate to what extent non-adiabatic temperature variations atthe surface of slowly rotating non-radially pulsating beta Cephei starsand slowly pulsating B stars affect silicon line profile variations. Weuse the non-adiabatic amplitudes of the effective temperature andgravity variation presented in Dupret et al. (\cite{Dupret02}), togetherwith a Kurucz intensity grid, to compute time series of line profilevariations. Our simulations point out that the line shapes do not changesignificantly due to temperature variations. We find equivalent widthvariations of at most two percent of the mean equivalent width. Weconfront our results with observational equivalent width variations andwith photometrically obtained effective temperature variations. Based onobservations obtained with the Swiss photometric telescope and with theESO/CAT telescope, at La Silla in Chile.
| Observational status and excitation mechanisms of beta-Cephei variables.
Not Available
| Nonadiabatic observables in beta Cephei models
Using results of linear nonadiabatic calculations for oscillations ofbeta Cephei star models, we calculate amplitudes and phases for light,color and radial velocity for the unstable modes of low harmonicdegrees, l. The nonadiabatic observables are the amplitude ratios andthe phase differences for various oscillating parameters. We constructtheoretical diagrams involving these observables as well as pulsationperiods and compare them with the stellar data. Balona & Stobie(1979) showed that the diagrams based on two-color photometric data maybe used to determine the l-value of an observed mode. Our use of resultsof nonadiabatic calculations improves their method. We show, inparticular, that the diagrams employing the satellite ultravioletmeasurements are the best for discrimination between the l = 0, 1 and 2cases. The clearest separation of the domains for the three l-valuesoccurs in the diagram making use of both photometric and the radialvelocity data. The observational points fall into three theoretricaldomains and an assignment of the l-value is unambiguous. A comparison ofthe theory with the observations is also made using the Period versusAmplitude ratios diagrams in various photometric systems. The agreementis very satisfactory and, in most cases, the l-value can be determined.The nonadiabtic observables are useful not only to determine l but alsothe radial order of the observed modes as well as for constrain meanstellar parameters. As an example we consider the case of delta Ceti-asingle mode beta Cephei star. Our results point to the significantmulticolor photometric and spectroscopic data for asteroseismology.
| A photometric study of β Cephei stars. II. Determination of the degrees L of pulsation modes
The wavelength dependence of photometric amplitudes is used as a meansof identifying the degrees l of pulsation modes of β Cephei starsstudied in an earlier paper. To this end, an expression for thephotometric amplitude of a non-rotating pulsating star is derived interms of the wavelength of the radiation received and the degree of thepulsation mode involved. The derivation differs from earlier derivationsin that the specific radiation intensity is considered instead of theoutward normal radiation flux. Furthermore, the effects of thenon-radial components of the Lagrangian displacement on the localsurface element of the star are taken into account. The angulardependence of the specific radiation intensity is determined by alimb-darkening function. The relative Lagrangian perturbation of thespecific radiation intensity is set equal to the relative Lagrangianperturbation of the outward normal radiation flux. The physicalparameters of the β Cephei stars are estimated by means ofcalibrations of photometric systems. From a comparison of thecalibrations of the Walraven, the Geneva, and the Stroemgren system forearly-type stars, it appears that the most reliable values are obtainedby means of the Walraven system. The influence of the uncertainties onthe physical parameters on the determination of the degrees l ofpulsation modes in β Cephei stars is examined. The expression forthe photometric amplitude of a pulsating star is used for thedetermination of the degree l of a pulsation mode by fitting curves ofthe wavelength dependences of theoretical photometric amplitudes forvarious degrees l to the wavelength dependence of observationallydetermined photometric amplitudes. In this way, values for the degrees lof most of the pulsation modes of the β Cephei stars considered arefound. It appears that not all β Cephei stars pulsate in at leastone radial mode and that multiperiodic β Cephei stars pulsate in avariety of combinations of pulsation modes.
| Mode identification with the moment method in four multiperiodic ? Cephei stars: KK Velorum, ? Eri, ? CMa, and V348 Normae
We present analyses of line profile variations obtained for fourmultiperiodic ? Cephei stars: KK Velorum, ? Eri, ? CMa, andV348 Normae. An identification of the modes has been attempted by meansof the moment method. This quantitative identification technique wasfirst proposed by Balona (1986) and was later refined by Aerts et al.(1992) in case of a monoperiodic pulsation in a slowly rotating star.Recently, Mathias et al. (1993) have proposed a generalisation of themoment method to multiperiodic pulsations. We apply their formalism toobtain the velocity parameters in the four mentioned stars. We havefound that KK Velorum, instead of being a monoperiodic variable,pulsates in two modes of which the second one has a period that is halfof the period of the already known mode. The Q-values for the two modesindicate that the main mode probably is a g-mode, while the second onedefinitely is a p-mode. Because we did not fully cover the overallbeat-period in ? Eri and ? CMa, we could treat only the mostimportant modes present in these stars. We found it very useful forthese cases to be able to combine photometric and spectroscopic results.The rapidly rotating star V348 Normae seems to pulsate in threehigh-degree modes with very large amplitudes, a confirmation of theresult obtained from a photometric analysis presented by Waelkens &Cuypers (1985).
| Beta Cephei stars from a photometric point of view
This is an observational review, with an emphasis on photometric dataand their interpretation. Two lists are presented, one containing BetaCephei stars and the other Beta Cephei suspects. These lists then serveas a basis for discussing such topics as the location of Beta Cepheistars in the observational and theoretical H-R diagrams, theevolutionary state of these stars, the period-luminosity andperiod-luminosity-color relations, and observational identification ofpulsation modes. The paper also includes references to recent workconnected with the theoretical discovery that an opacity mechanism isresponsible for the excitation of Beta Cephei-star pulsations. Finally,observational programs for verifying the consequences of this discoveryare suggested.
| Observational evidence for metallicity-dependence of the Beta Cephei instability strip
The southern early-B star HD 166540 is shown to be a probable BetaCephei star with principal pulsation period 0.23299 days. Additionalperiods are probably present. According to its colors in the Genevaphotometric system, the star is significantly hotter than the classicalBeta Cephei stars. All recently discovered Beta Cephei stars withgalactocentric distances significantly smaller than that of the sun arefairly blue, and the locus of the instability strip can depend onmetallicity. The hypothesis is discussed in terms of the recentproposition that the pulsations of Beta Cephei stars are due to aniron-opacity mechanism. The mechanism does not support the sometimesclaimed link between Beta Cephei stars and Be stars.
| Long-term photometry of variables at ESO. I - The first data catalogue (1982-1986)
This paper presents the catalog of photometric data in the Stromgrensystem obtained during the first four years (October 1982 - September1986) of the Long-Term Photometry of Variables (LTPV) program at ESO.The data are available in computer-readable form.
| The 69th Name-List of Variable Stars
Not Available
| Two new multiperiodic Beta Cephei candidates in the Scorpius complex
The discovery of two hitherto unknown Beta Cephei candidates isreported. The photometric variations of both stars indicate the presenceof at least three frequencies. The periods of HD 147985 are 0.13231 d,0.14493 d, and 0.15666 d; the periods of HD 15662 are 0.16890 d, 0.18861d, and 0.16978 d. Both variables belong to the Scorpius complex. Unlikethe Beta Cephei candidates discovered in NGC 6231 (Balona and Shobbrook,1983), they fit into the classical instability strip. It thus seemsunlikely that the peculiar position in the HR diagram of the variablesin NGC 6231 is related to the galactic location of the cluster. Thepossibility of a more extended instability strip is discussed in view ofthe results of recent surveys in the solar neighborhood. It is foundthat the actual instability strip closely parallels the main sequence.
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